1. Field of the Invention
The invention relates to a strap driving device for a strapping machine comprising a device for inserting, returning and tensioning a strap, the device having at least one pair of rollers through which the strap is drivably guided in and opposite to the insertion direction and is tensionable around the object in the strapping machine opposite to the insertion direction, and guide channel portions for the strap which lead toward the device for inserting, returning and tensioning the strap and away from said device, with the guide channel portions being provided with cheeks guiding the strap along both sides of its flat sides.
2. Background Art
A strapping machine is for example disclosed in DE 196 02 579 A1. Along with the components which are usually present in such machines such as machine frame, work table, and strap guide frame for guiding the strap around the object to be strapped in the form of a loose loop, the strapping machine comprises a strap driving device disposed underneath the work table. Said strap driving device comprises a combined device for inserting, returning and tensioning the strap.
Practically speaking, an insertion-return unit for inserting the strap into the strap guide frame and for returning the strap from the strap guide frame until the strap is disposed around the object to be strapped as well as a tensioning unit for tautening the strap around the object are provided to achieve this purpose, with the tensioning unit being in many cases dependent on the stack height.
The insertion-return unit comprises a pair of rollers, with the strap being guided through the roller gap thereof in such a way as to be drivable by one of the rollers in the insertion and return directions. The tensioning unit comprises another pair of rollers, with the strap also being guided through the roller gap of which so as to be tensionable, by means of at least one of the rollers, in the return direction about the object to be strapped in the strapping machine.
For guiding the strap through the strap driving device, there are finally provided guide channel portions which guide the strap supplied by a supply roller or an intermediate storage device to the tensioning unit, between the tensioning unit and the insertion-return unit and from the insertion-return unit in the direction of the strap guide frame on the work table. The guide channel portions are in each case formed by cheeks guiding the strap along both sides of the flat sides, the cheeks being embodied as webs or lateral surfaces of larger prismatic bodies.
A major problem in the operation of strapping machines is the trouble-free handling of the strap which, in order to achieve high cycle times for the strapping process, needs to be guided through the strapping machine at high speeds before it is suddenly reversed in its direction of movement so as to come to a stop. Owing to the flexible nature of the strap or due to a wear of the machine or of the strap along the transport path, the strap may get stuck or tangled up along the transport path. The strap driving device is particularly susceptible to this kind of problem due to the fact that the drive rollers of the insertion-return unit and of the tensioning unit directly act on the strap in the strap driving device. The problem in this regard is that the various driving components of the strap driving device are arranged in a closely nested manner, making the strap very hard to access. In the event of heavy problems such as a so-called “Z-folding” of the strap where three strap layers are disposed on top of one another over a short distance, the strap may become “jammed” in the guide channel portions in such a way that it can no longer be pulled out. In order to resolve this problem in known strap driving devices, one or several cheeks of the guide channel portions or even rollers of the various driving units need to be dismounted. This, of course, results in a high machine downtime which may not only affect the strapping machine but, in the worst case, also an entire production line for print media, for example.